Logical box model properties: margin, padding, and border all get an upgrade for more use cases. Think of how we have margin-left now — the “left” part doesn’t make much sense when we switch directions. Now, we’ll have margin-inline-start for that. The full list is margin-{block,inline}-{start,end}, padding-{block,inline}-{start,end} and border-{block,inline}-{start,end}-{width,style,color}. Here’s Rachel Andrew with Understanding Logical Properties And Values.

Environment variables (i.e. env(safe-area-inset-top);): Apple introduced “the notch” with the iPhone X and dropped some proprietary CSS for dealing with it. The community quickly stepped in and now we have env() for browsers to ship stuff like this.